1. Field of the Invention
This invention relates particularly to a high-pressure fuel injection pipe for a Diesel engine which pipe has a relatively small pipe diameter of not greater than about 30 m/m and is disposed in the proximity of the Diesel engine as a fuel supply path.
2. Description of the Related Art
High-pressure fuel injection pipes for a Diesel engine employ generally the following construction as their principal portions are depicted in FIG. 5, to completely prevent scatter and leak of a fuel outside the engine. Outer pipes 12-1 and 12-2 as discrete bodies are fitted to the outer peripheral side of an injection pipe main body 11 in such a fashion that they can move in an axial direction while their opposed end faces keep a predetermined space 14 between them. An interconnecting cylinder body 13 is so disposed near the mutual end portions of the outer pipes 12-1 and 12-2 as to bridge both outer pipes 12-1 and 12-2. This cylinder body 13 is thermally welded (W-1 and W-2) at both end portions thereof to positions of the inner end portions of the outer pipes 12-1 and 12-2 to define a double wall structure.
In the injection pipe having such a construction, the outer pipes 12-1 and 12-2 must be put and assembled at the predetermined positions of the injection pipe main body 11. Under this state, the interconnecting cylinder body 13 must be then disposed in such a manner as to bridge both outer pipes 12-1 and 12-2. Thereafter, both end portions of the interconnecting cylinder body 13 must be thermally welded (W-1, W-2) to the outer pipes 12-1 and 12-2 by brazing or welding. Consequently, the thermal welding process step is troublesome to conduct. In addition, this heating step invites deterioration of the mechanical strength due to heating of the fuel injection main body 11 positioned near the thermal welding portion, and the drop in the corrosion resistance due to the heat loss of the corrosion-resistant process coating of the outer pipes 12-1 and 12-2.
In other words, the thermal welding process step must be carried out for two positions after the fuel injection pipe is assembled and completed substantially in the form of a product, and a step of repairing the coating such as Zn plating, that is damaged by thermal welding, becomes necessary. On the other hand, the fuel injection pipe main body 11 is produced by repeatedly stretching and heat-treating a mother pipe comprising a steel pipe or a stainless steel pipe. A final stretched pipe is further heat-treated to obtain a crystal grain size and various properties, such as hardness, suitable for the injection pipe. Thermal welding W-1 and W-2 invites embrittlement or changes the structure to an annealed structure due to coarsening of the crystal grain size in the case of the steel pipe, and invites deterioration due to precipitation of Cr--C in the case of the stainless steel pipe. Such deterioration of the mechanical strength and vibration from the engine during the practical use result in the occurrence of cracks in the peripheral wall in the course of the fuel injection pipe for a long time. In consequence, scatter and leak of the fuel in the atomized state outside the engine might occur, and safety measures must be further taken.
To cope with this problem, the applicant of this invention proposed previously a high-pressure fuel injection pipe that can eliminate the troublesome thermal welding step on the peripheral wall surface on the side of the injection pipe main body positioned near the bridging portion of the interconnecting cylinder, the troublesome repairing step of the corrosion-resistant coating and deterioration of the mechanical strength, can prevent the scatter and leak of the fuel to the outside, can insure more reliably the counter-measure against the fire, and can be used for a long period (Japanese Patent Laid-Open No. 5-296121).
This high-pressure fuel injection pipe has the following construction. The fuel injection main body has an assembly of connection head portions formed at both end portions and fastening nuts disposed at the back of the connection head portions, respectively. Outer pipes each comprising a discrete body are put to the outer peripheral side of the fuel injection main body in such a fashion that they can move in an axial direction while keeping a gap between their opposed end faces. The mutual outer end portions of these outer pipes are meshed with the fastening nuts through seal ring members, respectively. A short plate-like interconnecting cylinder body is so disposed near the mutual inner end portions of the outer pipes as to bridge these outer pipes. This high-pressure fuel injection pipe is assembled in the following way. One of the outer pipes, that is positioned on the side of the connection head portion, and the interconnecting cylinder body on the same side as the former are meshed with one of the fastening nuts through one of the fastening nuts that is assembled in advance near the outer end portion of the outer pipe. Under this state, these members are brazed. Next, the other outer pipe, that is similarly meshed with the other fastening nut through the other seal ring member, is fitted from the other end of the injection pipe main body. Thereafter, the other outer pipe and the other fastening nut are moved towards the connection head portion at one end that is already formed. Under this state, the other connection head portion is then formed. Furthermore, the other fastening nut and the other out pipe are returned to the respective normal positions, and then the interconnecting body is fitted to bridge the outer pipes through the seal ring member.
The prior art technology described above employs brazing to connect and fix one of the outer pipes to the interconnecting cylinder body. When this brazing is used for bonding, the troublesome surface treatment must be carried out afterwards to prevent the heat loss of the surface-treatment resulting from brazing. When both ends of the outer pipe are sealed by the sealing member, the seal position is likely to deviate from the normal position because the outer pipes are freely movable in the axial direction.